Pipelines are normally constructed by joining the free facing ends of two adjacent, longitudinally aligned pipe sections to form a cutback; and winding protective sheeting about the cutback. Pipelines are made of pipe sections joined to cover distances of hundreds of kilometers. Each pipe section is normally 12 meters (39 feet, 4.4375 inches) long, with a relatively large diameter ranging between 0.2 meters (7.875 inches) and 1.5 meters (4 feet, 11.0625 inches), and comprises a metal cylinder; a first coating of polymer material to protect the metal cylinder; and possibly a second coating of Gunite or concrete, which serves as ballast and is not always needed. To weld the metal cylinders together, the free opposite ends of each pipe section have no first or second coating; and the pipe sections are joined either at land-based installations or on board laying vessels configured to lay the pipeline as the pipeline is being built.
Joining the pipe sections comprises welding the metal cylinders, normally in a number or quantity of passes; and restoring the first and second coating (if any). Once an annular weld bead is formed between each two adjacent metal cylinders, the cutback extends astride the annular weld bead, along a bare portion with no first or second coating. In other words, the cutback is substantially defined by the free ends of the pipe sections, extends axially between two end portions of the first coating, and must be covered with a protecting coating to prevent corrosion.
Restoring the first coating along the cutback is known as ‘field joint coating’, and normally comprises coating the cutback with three coats of polymer material to protect and ensure adhesion of the coats to the metal cylinders. More specifically, restoring the first coating along the cutback comprises heating (e.g., induction heating), the cutback to a temperature of 250° C.; spraying the cutback with powdered epoxy resin (FBE—fusion bonded epoxy) which, in contact with the cutback, forms a relatively thin first coat or ‘primer’; spraying the cutback, on top of the first coat, with a modified copolymer, which acts as adhesive and, in contact with the first coat, forms a relatively thin second coat; and applying a third so-called ‘top coat’, which also extends partly over the first coating. The second coating, if any, is then also restored.
Welding, non-destructive weld testing, and restoring the first and second coating are all performed at work stations equally spaced along the path of the pipe sections (or the pipeline being built, if the pipe sections are joined to this), so the pipe sections are advanced in steps and stopped for a given length of time at each work station.
One known method of applying the third coat to restore the first coating comprises extruding and simultaneously winding thick protective sheeting about the cutback, as described in the Applicant's PCT Patent Application No. WO 2008/071773, in European Patent Application No. EP 1,985,909, in PCT Patent Application No. WO 2010/049353 and in PCT Patent Application No. WO 2011/033176. In the methods described, the protective sheeting is applied by an extrusion port mounted on a carriage, which runs along an annular path about the longitudinal axis of the pipeline; and the polymer material is plastified by a plastifying device located close to the pipeline and either selectively connectable to or hose-connected to the extrusion head. Both methods of feeding the soft polymer material to the extrusion head have drawbacks, on account of the physical characteristics of the polymer material, which, to remain soft, must be heated to within a given temperature range.